Originally Posted by bigmac
The tank is vented...shouldn't be any negative or positive pressure buildup.
are you familiar with the setup on the 02 xstar? Attached is a picture of the top of my tank. As we all know, A is the big hose for filling the tank with fuel, C and D are the high pressure line to the fuel rail, and the return line from the fuel rail, and B, the hose that is disconnected from the nearby nipple, is the only 'vent' hose I can find coming from the tank. The only problem is that hose B runs parallel to hose A the entire time, and attaches to a nipple that is clearly visible in the fuel inlet opening on the TANK side of the fuel cap. When I close my fuel cap, block the open nipple you see on the tank, and blow into hose B, the tank pressurizes. If I open the gas cap and blow into hose B, the air escapes there, from the spot where hose B attaches which as I said is on the tank side of the cap. If I repeat the test with the fuel cap on and unplug the nipple on top of the tank, of course the air escapes there rather than the tank pressurizing. Point being, as my system is designed, there is not a vent to allow air to escape unless you leave the gas cap loose, or unless your gas cap o-ring isn't sealing.
I did not repeat this test drawing a suction on the hose as I was afraid I'd somehow end up with a mouthfull of gas fumes, but I do wonder what provisions the system has to allow air INTO the tank to replace burned fuel...I have to believe somewhere there is a way for air to get into the tank.
The only purpose for the vent hose B in my system is to allow for air to escape during refueling, it would burp and gurgle while filling if B wasn't there to route air to the entrance of the fuel filler, air that is being displaced by incoming fuel...
Once the cap is screwed on tight hose B is no longer venting anything..
It is possible that there is a one way check-valve somewhere that allows air into, but not out of, the tank. Possibly even part of the metal round piece that C attaches to that is part of the FPR on the newer models. Haven't tracked down if this is the FPR on my system, but I'm accustomed to FPR's being near the fuel rail..
Last time I refueled I didn't notice any hiss. The time before, when it did hiss, I think the temp had risen 20 degrees or more from the time I refueled to the time i removed the cap, and we had driven the boat only a little bit. I suspect the system had pressurized from expansion of fuel, and i suspect my system is also designed not to let any vapors or air out of the tank.
Seems like all mastercraft systems would share this ventless design, but I only know what is going on with my boat..
If anyone can point me to a vent source or hose that I'm unaware of, or a through hull fitting that vents the fuel system like I have on my early 90's scarab I would really appreciate it..
This seems pretty cut and dried (that my tank isn't vented), but I'm being told otherwise by some very credible members on this forum and as such I need to get to the bottom of whether my system is faulty by not being vented, or whether the info I'm receiving is faulty..